<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        // console.log(a);//报错
    </script>
    <script>
        console.log(a);
        var a = 10;
        console.log(a);
        // console.log(b);
    function fn(){
        console.log(b)
        var b = 20;
        console.log(b)
        console.log(a);
    }
    fn();
    console.log(b);
    console.log(a);
    </script>
    <script>
        console.log(a);//可以获取
    </script>
</body>

</html>